The present invention relates to an optical recording medium and more particularly to a high-density optical recording medium having a track width smaller than an optical spot diameter.
An example of a medium for performing high-density (narrow track) recording is disclosed in, for example, JP-A-6-176404. According to this example, in an optical recording medium having grooves and lands which are formed on a substrate and information recording areas which are formed in association with both the groove and the land, prepits are disposed on a virtual extension line of the boundary between a groove and a land. In particular, the prepits are located on only one side of any specific position of the center line of each groove.
With this construction, recording information is formed on both the groove and the land, the prepits have charge of address data representative of the recording areas and one prepit is used in common to a pair of adjacent groove and land to provide address data therefor. When the technique as above is applied to, for example, a phase change recording medium or a magneto-optical recording medium, interference of information (crosstalk) between adjacent lands or grooves due to the optical interference effect within an optical spot can be prevented, thereby permitting narrowing of track.
On the other hand, in the prepit area free from the optical interference effect, the address data can be common to the paired groove and land and the effective track pitch can be increased to reduce crosstalk.
In the example of JP-A-6-176404, however, the disposition of the prepit area is offset on one side of the center line of the groove or land, so that when an optical spot is caused to track a groove or a land, a tracking error (tracking offset) increases, making it difficult to perform high-density recording in which the track pitch is narrowed.
The present invention achieves elimination of the above problems and it is a first object of the present invention to provide an optical recording medium which can suppress the tracking offset to a value or level which is sufficiently low for the practical use and permit efficient disposition of address data even when recording is effected on both the groove and the land.
A second object of the present invention is to provide a high-density optical recording medium which can ensure simple mastering and easy replica preparation and can permit decoding even when a readout error takes place.
To accomplish the above first object, the following expedients are employed.
(1) Grooves and lands are formed on a substrate of a recording medium, information recording areas are formed in association with both the groove and the land, and prepits are disposed on a virtual extension line of the boundary between a groove and a land.
The disposition of prepits satisfies the following requirements (a) to (c) at the same time.
(a) Prepits are located on both sides of a virtual extension of the center line of a groove;
(b) Prepits are located on both sides of a virtual extension of the center line of a land;
(c) Prepits are not located on the both sides of any specific position of the center line of a groove; and
(d) Prepits are not located on the both sides of any specific position of a land.
With this construction, the arrangement of prepits is not offset on either one side of a virtual extension of the center line of the groove or the land to ensure that tracking offset hardly occurs and the prepits do not exist on both sides of any specific position of the center line of the groove or the land to prevent interference of prepit information between adjacent tracks from taking place within a reproduced spot so as to permit high-density narrow track recording.
(2) When prepits are disposed in the circumferential direction such that those on one side of a groove are not discriminative from those on the other side or those on one side of a land are not discriminative from those on the other side, at least consecutive two dispositions of prepits associated with the groove or the land are made to be different from each other to provide the same disposition of prepits periodically every two dispositions.
As the other option,
(3) A groove associated with at least one pair of pits disposed on both sides of the center line of the groove in a prepit area and an adjacent groove not associated with pits disposed on both sides of the center line of this groove within the prepit area are disposed alternately in the radial direction.
Through this, by merely reproducing the pits, prepits associated with the groove can be discriminated from those associated with the land to improve reliability of information recording reproduction.
(4) Either one of synchronous information and address data is represented by prepits disposed on either one of the both sides of a groove.
As the other option,
(5) Only one of synchronous information and address data is represented by prepits arranged on one side of a groove and both the synchronous information and the address data are represented by prepits arranged on the other of the both side of the groove.
Through this, address data can be reproduced under accurate synchronization. In addition, since the phase margin between prepits on the both sides can be extended, fabrication of a recording medium can be facilitated.
(6) The groove and the prepit have the same depth which is 70 nm or less. More preferably, the depth is 40 nm or more and 60 nm or less.
With this construction, an advantage of suitable crosstalk cancellation can be obtained between the groove and the land and besides an excellent tracking servo signal can be obtained. Formation and fabrication of the recording medium can be facilitated. With the groove depth exceeding 70 nm, the formation of the groove is difficult to achieve. When the groove depth is about 50 nm, the tracking servo is maximized and with the groove depth being about 50xc2x110 nm, substantially the same effect can be attained.
(7) The groove and the land have substantially the same width which is between 0.3 xcexcm and 0.75 xcexcm.
With this construction, excellent tracking is compatible with high-density recording. If the groove and land have a width of not greater than 0.3 xcexcm, then two of the groove and land will be confined within one optical spot and an excellent tracking signal cannot be obtained. On the other hand, if the groove and land have a width exceeding 0.75 xcexcm, then effective high-density recording cannot be permitted.
(8) Of prepits, the smallest one has a diameter which is smaller than a width of each of the groove and the land. More preferably, the diameter is in the range from 0.25 xcexcm to 0.55 xcexcm.
Through this, an excellent prepit signal can be obtained without crosstalk. With the diameter being not greater than 0.25 xcexcm, the prepit signal decreases in the extreme and with the diameter exceeding 0.55 xcexcm, crosstalk is generated.
In the present invention, prepits are arranged on the both sides of a virtual extension line of the center line of a groove or a land in the optical spot scanning direction. Consequently, offset is decreased to make the tracking offset hardly occur and the prepits do not exist on the both sides of any specific position of the center line of the groove or the land to ensure that interference of prepit information between adjacent tracks within a reproduced spot can be prevented, and high-density narrow track recording can be permitted.
Further, even in the presence of tracking offset, the amount of tracking offset can be detected accurately by comparing amplitudes of signals representative of prepits on the both sides. Accordingly, by feedback-controlling the information indicative of a comparison result to a scanning unit, the tracking offset can be suppressed.
At a portion between a groove and a prepit area, between a land and a prepit area or between prepit areas, a gap takes place when a prepit train on a virtual extension line of the boundary between a groove and a land shifts to a prepit train on an adjacent virtual extension line. The aforementioned JP-A-6-176404, however, does not take the gap into consideration. Accordingly, in the absence of the gap or with the gap being very short, mastering of the substrate cannot be proceeded with by one-beam cutting and requires two-beam cutting. Further, during replica preparation, injection must be applied to a steep pattern, leading to a decrease in yield. In addition, during reproduction of signals, tolerance to distortion of the reproduced spot and the tracking offset is decreased and a readout error is liable to occur.
To accomplish the second object, the following expedients are employed.
(1) Grooves and lands are formed on a substrate and prepits are arranged on a virtual extension of the boundary between a groove and a land. In particular, the prepits are disposed on both sides of an extension of the center line of a groove or a land and therefore, the optical axis of a laser beam must be moved during cutting. An acoustic-optical deflector (AOD) is used to change the optical axis. But it takes a time for the AOD to cause the optical axis to reach a desired optical axis position after transmitting a signal for optical axis change and when a modulated laser beam is irradiated along the intact optical axis, pits are formed obliquely on the substrate. Accordingly, no pattern is formatted between the groove or the land and the succeeding prepits to provide a gap and an acoustic-optical modulator (AOM) is cut off corresponding to the gap to prevent laser irradiation and pit drawing. Thus, the substrate can be fabricated with a simple cutting machine. In addition, since a number of unevennesses are not formed on a narrow area on the substrate, the yield during preparation of replica can be increased.
(2) In the disposition in which prepits are arranged on a virtual extension of the boundary between a groove and a land, when the disposition of a prepit train on one side of a virtual extension of the boundary between the groove and the land is exchanged with the disposition of a prepit train on the other side or vice versa, the trailing edges of prepit trains on the respective one sides are aligned with each other in the radial direction of the substrate. The succeeding pit trains are spaced from those trailing edge positions in the circumferential direction or the recording/reproducing direction and the trailing edges of the succeeding pit trains are aligned with each other similarly. When the formed gap meets the recording rule, the substrate as a whole can be formatted conveniently and portions devoid of pits can be collected at a specified area on the substrate, thereby solving problems involved in cutting and replica preparation for reasons described previously.
(3) Radially adjacent pit trains each having only original information pits cannot be aligned with each other at the trailing edge in the radial direction. Accordingly, new pits are added to ensure the alignment of the trailing edges in the radial direction while observing the rule during recording.
(4) In the shift of the disposition of a pit train from one side to the other as described in the above (2), leading edges of pits in the disposition on the other side can be aligned in the radial direction to solve the problems involved in cutting and replica preparation for the same reasons set forth in the (2). In particular, from the standpoint of signal reproduction, a synchronous signal is allotted to pit information immediately after the shift of the pit train so that decision of a channel bit at the specified position may be thought much of, thereby ensuring that the tolerance to the leading edge position can be increased and a possibility that erroneous reading of important data of, for example, address at the position immediately before the shift of a pit train can be decreased.